"折合市制" can be translated as "converted to the Chinese municipal measurement system" or more concisely as "converted to the municipal system (of units)" in contexts where it's clear that "municipal" refers to the traditional Chinese system of measurement.
However, if you're specifically referring to converting to the old or traditional Chinese system of weights and measures (often contrasted with the metric system), you might also say "converted to the traditional Chinese measurement system" or "converted to the old Chinese units."
In most practical or technical contexts, a more straightforward translation like "converted to the municipal (unit) system" (with a possible explanation or context provided) would suffice, especially if the audience is familiar with the distinction between metric and traditional Chinese units.
For example:
"This measurement has been converted to the municipal system."
(With additional context:) "This measurement, originally in metric units, has been converted to the traditional Chinese (municipal) measurement system for ease of comparison in local contexts."